Over the last few decades desalination technologies have been used increasingly throughout the world to produce drinking water from brackish groundwater and seawater. In the early 1950's there were about 225 land-based desalination plants worldwide, and in the mid-1990's, there are over 4000. Nevertheless, in many areas of the world, even next to oceans, useable water remains in short supply. This is particularly true for agricultural use. The problem is that desalination is still expensive, and consequently, large quantities of desalinated water are cost prohibitive and are not available. The present invention is directed to creating large quantities of agriculturally-useable, desalinated water at a competitively affordable cost.
The present invention uses reverse osmosis technology. Conventional reverse osmosis systems require pumps to create operational pressures. Reverse osmosis membranes are housed in pressure containers in order to receive the incoming pressurized water. Operational electrical cost for the pumps becomes a key factor in ultimate water cost. Because the cost of creating the pressure is so significant, conventional systems connect multiple reverse osmosis elements in series so that brine water from the last element flows as input to the next element. Although any pressure drop across an element is minimal, salt concentration increases because some of the original water has permeated through the last element as product water. As salt concentration for downstream elements due to precipitation of salts increases, scaling of the elements increases. Over time, this results in frequent maintenance, that is, cleaning of elements or replacement of elements. Because maintenance cost becomes significant, water pretreatment is done to try to reduce fouling and scaling. The problem, however, is that the cost of pretreatment materials becomes high. All these costs (electrical, maintenance, pretreatment, element replacement) add together to make the ultimate cost of the desalinated water high.
Although, as indicated, conventional reverse osmosis technology is used to provide drinking water in locations that can afford it, costs as a result of the above factors are prohibitive for obtaining higher quantity and higher quality agriculturally-useable water. In this regard, it is noted that agriculturally-useable water must be purer than drinking water. This is true since any salt remaining in the water accumulates on land. That is, after a few years of irrigation of fertile land with water which is acceptable for drinking but which nevertheless has some salt content remaining, the once fertile land will become so alkaline that it no longer will support crops and will become arid. Thus, as indicated, there remains a tremendous need for desalinated, agriculturally-pure water that is available in adequate quantities at affordable prices.
To reduce the need for operational electricity, the present invention uses gravity to create operational pressures. Others have conceptualized the use of gravity. The apparatus disclosed in UK patent application GB2,068,774 is of most interest. In it, gravity is used to provide hydrostatic pressure as operational pressure for reverse osmosis elements located in a subterranean cavity. The brine waste water is forced most of the way back to the surface by the hydrostatic pressure. The drinking water is pumped to the surface. This disclosure does not understand, however, the need for and, consequently, does not address desalination to obtain agriculturally-useable water.